Optical differential low-noise receivers and related methods

ABSTRACT

Low-noise optical differential receivers are described. Such differential receivers may include a differential amplifier having first and second inputs and first and second outputs, and four photodetectors. A first and a second of such photodetectors are coupled to the first input of the differential amplifier, and a third and a fourth of such photodetectors are coupled to the second input of the differential amplifier. The anode of the first photodetector and the cathode of the second photodetector are coupled to the first input of the differential amplifier. The cathode of the third photodetector and the anode of the fourth photodetector are coupled to the second input of the differential amplifier. The optical receiver may involve two stages of signal subtraction, which may significantly increase noise immunity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/411,391, entitled “DIFFERENTIAL, LOW NOISE HOMODYNE RECEIVER,” filedMay 14, 2019, which claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 62/793,327, entitled“DIFFERENTIAL, LOW-NOISE HOMODYNE RECEIVER,” filed on Jan. 16, 2019,each of which is hereby incorporated herein by reference in itsentirety.

BACKGROUND

Photodetectors are sensors configured to generate electric signalsresponsive to reception of light. In optical communications,photodetectors are often used to detect optical signals. For example, aphotodetector can be connected to an end of an optical fiber to detectoptical signals traveling down the fiber.

SUMMARY OF THE DISCLOSURE

Some embodiments relate to an optical receiver comprising a differentialamplifier having first and second inputs and first and second outputs;first and second photodetectors coupled to the first input of thedifferential amplifier; and third and fourth photodetectors coupled tothe second input of the differential amplifier.

In some embodiments, an anode of the first photodetector is coupled to acathode of the second photodetector.

In some embodiments, the anode of the first photodetector and thecathode of the second photodetector are coupled to the first input ofthe differential amplifier.

In some embodiments, a cathode of the third photodetector is coupled toan anode of the fourth photodetector.

In some embodiments, the cathode of the third photodetector and theanode of the fourth photodetector are coupled to the second input of thedifferential amplifier.

In some embodiments, the first, second, third and fourth photodetectorsare formed monolithically on a common substrate.

In some embodiments, the common substrate comprises a silicon substrate.

In some embodiments, the first, second, third and fourth photodetectorsare disposed within an area of 0.1 mm² on the substrate.

In some embodiments, the first, second, third and fourth photodetectorshave equal responsivities.

In some embodiments, the first, second, third and fourth photodetectorsare photodiodes.

In some embodiments, the optical receiver further comprises ananalog-to-digital converter coupled to the first and second outputs ofthe differential amplifier.

In some embodiments, the optical receiver further comprises a photoniccircuit configured to provide: a first optical signal to the first andthird photodetectors, and a second optical signal to the second andfourth photodetectors.

In some embodiments, the photonic circuit is configured to generate thefirst and second optical signals by combining a modulated optical signalwith a reference optical signal.

Some embodiments relate to a method for receiving an input signal, themethod comprising combining the input signal with a reference signal toobtain first and second optical signals; detecting the first opticalsignal with a first photodetector and with a second photodetector anddetecting the second optical signal with a third photodetector and witha fourth photodetector to produce a pair of differential currents; andproducing a pair of amplified differential voltages using the pair ofdifferential currents.

In some embodiments, detecting the first optical signal with the firstphotodetector and with the second photodetector and detecting the secondoptical signal with the third photodetector and with the fourthphotodetector to produce the pair of differential currents comprises:producing a first photocurrent with the first photodetector; producing asecond photocurrent with the second photodetector; producing a thirdphotocurrent with the third photodetector; producing a fourthphotocurrent with the fourth photodetector; and subtracting the firstphotocurrent from the third photocurrent and subtracting the secondphotocurrent from the fourth photocurrent.

In some embodiments, combining the input signal with the referencesignal comprises combining the input signal with the reference signalwith a directional coupler.

In some embodiments, producing the pair of amplified differentialvoltages using the pair of differential currents comprises producing thepair of amplified differential voltages using the pair of differentialcurrents with a differential operational amplifier.

Some embodiments relate to a method for fabricating an optical receiver,the method comprising: fabricating first, second, third and fourthphotodetectors; and fabricating a differential operational amplifierwith first and second inputs and first and second outputs such that thefirst and second photodetectors are coupled to the first input and thethird and fourth photodetectors are coupled to the second input.

In some embodiments, fabricating the first, second, third and fourthphotodetectors comprises fabricating the first, second, third and fourthphotodetectors on a first substrate; and fabricating the differentialoperational amplifier comprises fabricating the differential operationalamplifier on a second substrate and bonding the first substrate to thesecond substrate.

In some embodiments, bonding the first substrate to the second substratecomprises wire bonding the first substrate to the second substrate orflip-chip bonding the first substrate to the second substrate.

In some embodiments, the method further comprises fabricating a photoniccircuit configured to provide a first optical signal to the first andthird photodetectors and a second optical signal to the second andfourth photodetectors.

In some embodiments, fabricating the first, second, third and fourthphotodetectors comprises fabricating the first, second, third and fourthphotodetectors on a first substrate, and fabricating the differentialoperational amplifier comprises fabricating the differential operationalamplifier on the first substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1 is a circuit diagram illustrating an example of a differentialoptical receiver, in accordance with some non-limiting embodiments.

FIG. 2 is a schematic diagram illustrating a photonic circuit that maybe coupled with the differential optical receiver of FIG. 1 , inaccordance with some non-limiting embodiments.

FIG. 3A is a schematic diagram illustrating a substrate including aphotonic circuit, photodetectors and a differential operationalamplifier, in accordance with some non-limiting embodiments.

FIG. 3B is a schematic diagram illustrating a first substrate includinga photonic circuit and photodetectors, and a second substrate includinga differential operational amplifier, where the first and secondsubstrates are flip-chip bonded to each other, in accordance with somenon-limiting embodiments.

FIG. 3C is a schematic diagram illustrating a first substrate includinga photonic circuit and photodetectors, and a second substrate includinga differential operational amplifier, where the first and secondsubstrates are wire bonded to each other, in accordance with somenon-limiting embodiments.

FIG. 4A is a flowchart illustrating an example of a method forfabricating an optical receiver, in accordance with some non-limitingembodiments.

FIGS. 4B-4G illustrate an example of a fabrication sequence for anoptical receiver, in accordance with some non-limiting embodiments.

FIG. 5 is a flowchart illustrating an example of a method for receivingan optical signal, in accordance with some non-limiting embodiments.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that some conventionaloptical receivers are particularly susceptible to noise generated fromvoltage supplies, to noise arising from the fact that photodetectorsinevitably produce dark currents, and to other forms of noise. Thepresence of noise reduces the signal-to-noise ratio, and therefore, theability of these photodetectors to accurately sense incoming opticalsignals. This can negatively affect the performance of the system inwhich these photodetectors are deployed. For example, this cannegatively affect the system's bit error rate and power budget.

The inventors have developed optical receivers with reducedsusceptibility to noise. Some embodiments of the present application aredirected to optical receivers in which both the optical-to-electricconversion and the amplification are performed in a differentialfashion. In the optical receivers described herein, two separate signalsubtractions take place. First, the photocurrents are subtracted fromone another to produce a pair of differential currents. Then, theresulting differential currents are further subtracted from each otherto produce an amplified differential output. The inventors haverecognized and appreciated that having an optical receiver involvingmultiple levels of signal subtraction results in multiple levels ofnoise cancelation, thus substantially reducing noise from the system.This can have several advantages over conventional optical receivers,including wider dynamic range, greater signal-to-noise ratio, largeroutput swing, and increased supply-noise immunity.

Optical receivers of the types described herein can be used in a varietyof settings, including for example in telecom and datacom (includinglocal area networks, metropolitan area networks, wide area networks,data center networks, satellite networks, etc.), analog applicationssuch as radio-over-fiber, all-optical switching, Lidar, phased arrays,coherent imaging, machine learning and other types of artificialintelligence applications, as well as other applications. In someembodiments, optical receivers of the types described herein may be usedas part of a photonic processing system.

FIG. 1 illustrates a non-limiting example of an optical receiver 100, inaccordance with some non-limiting embodiments of the presentapplication. As shown, optical receiver 100 includes photodetectors 102,104, 106 and 108, though other implementations include more than fourphotodetectors. Photodetector 102 may be connected to photodetector 104,and photodetector 106 may be connected to photodetector 108. In someembodiments, the anode of photodetector 102 is connected to the cathodeof photodetector 104 (at node 103), and the cathode of photodetector 106is connected to the anode of photodetector 108 (at node 105). In theexample of FIG. 1 , the cathodes of photodetectors 102 and 108 areconnected to voltage supply V_(DD) and the anodes of photodetectors 104and 106 are connected to the reference potential (e.g., to ground). Theopposite arrangement is also possible in some embodiments. The referencepotential may be at a potential equal to zero or having any suitablevalue, such as —V_(DD). V_(DD) may have any suitable value.

Photodetectors 102-108 may be implemented in any of numerous ways,including for example with pn-junction photodiodes, pin-junctionphotodiodes, avalanche photodiodes, phototransistors, photoresistors,etc. The photodetectors may include a material capable of absorbinglight at the wavelength of interest. For example, at wavelengths in theO-band, C-band or L-band, the photodetectors may have an absorptionregion made at least in part of germanium, by way of a non-limitingexample. For visible light, the photodetectors may have an absorptionregion made at least in part of silicon, by way of another non-limitingexample.

Photodetectors 102-108 may be integrated components formedmonolithically as part of the same substrate. The substrate may be asilicon substrate in some embodiments, such as a bulk silicon substrateor a silicon-on-insulator. Other types of substrates can also be used,including for example indium phosphide or any suitable semiconductormaterial. To reduce variability in the characteristics of thephotodetectors due to fabrication tolerances, in some embodiments, thephotodetectors may be positioned in close proximity to one another. Forexample, the photodetectors may be positioned on a substrate within anarea of 1 mm² or less, 0.1 mm² less or 0.01 mm² or less.

As further illustrated in FIG. 1 , photodetectors 102-108 are connectedto a differential operational amplifier 110. For example, photodetectors102 and 104 may be connected to the non-inverting input (“+”) of DOA 110and photodetectors 106 and 108 may be connected to the inverting input(“−”) of DOA 110. DOA 110 has a pair of outputs. One output is invertingand one output is non-inverting.

In some embodiments, as will be described in detail in connection withFIG. 2 , photodetectors 102 and 106 may be arranged to receive the sameoptical signal “t” and photodetectors 104 and 108 may be arranged toreceive the same optical signal “b.” In some embodiments, photodetectors102-108 may be designed to be substantially equal to each other. Forexample, photodetectors 102-108 may be formed using the same processsteps and using the same photomask patterns. In these embodiments,photodetectors 102-108 may exhibit substantially the samecharacteristics, such as substantially the same responsivity (the ratiobetween the photocurrent and the received optical power) and/orsubstantially the same dark current (the current generated when nooptical power is received). In these embodiments, the photocurrentsgenerated by photodetectors 102 and 106 responsive to reception ofsignal t may be substantially equal to each other. Such photocurrentsare identified as “i_(t)” in FIG. 1 . It should be noted that, due tothe orientations of photodetectors 102 and 106, the photocurrentsgenerated by photodetectors 102 and 106 are oriented in oppositedirections. That is, the photocurrent of photodetector 102 is directedtowards node 103 and the photocurrent of photodetector 106 is orientedaway from node 105. Furthermore, the photocurrents generated byphotodetectors 104 and 108 responsive to reception of signal b may besubstantially equal to each other. Such photocurrents are identified as“i_(b).” Due to the orientations of photodetectors 104 and 108 relativeto each other, the photocurrents generated by photodetectors 104 and 108are oriented in opposite directions. That is, the photocurrent ofphotodetector 108 is directed towards node 105 and the photocurrent ofphotodetector 104 is oriented away from node 103.

In view of the orientations of the photodetectors, a current withamplitude i_(t)-i_(b) emerges from node 103 and a current with amplitudei_(b)-i_(t) emerges from node 105. Thus, the currents have substantiallythe same amplitudes, but with opposite signs.

Photodetectors 102-108 may produce dark currents. Dark currents aretypically due to leakage and arise from a photodetector regardless ofwhether the photodetector is exposed to light or not. Because darkcurrents arise even in the absence of incoming optical signals, darkcurrents effectively contribute to noise in the optical receiver. Theinventors have appreciated that the negative effects of these darkcurrents can be significantly attenuated thanks to the currentsubtraction described above. Thus, in the example of FIG. 1 , the darkcurrent of photodetector 102 and the dark current of photodiode 104substantially cancel out one another (or at least are mutuallysubstantially reduced), and so do the dark currents of photodetector 106and 108. Consequently, noise due to the presence of the dark currents isgreatly attenuated.

FIG. 2 illustrates a photonic circuit 200 arranged for providing twooptical signals to photodetectors 102-108, in accordance with somenon-limiting embodiments. Photonic circuit 200 may comprises opticalwaveguides for routing the optical signals to the photodetectors. Theoptical waveguides may be made of a material that is transparent or atleast partially transparent to light at the wavelength of interest. Forexample, the optical waveguides be made of silicon, silicon oxide,silicon nitride, indium phosphide, gallium arsenide, or any othersuitable material. In the example of FIG. 2 , photonic circuit 200includes input optical waveguides 202 and 204 and couplers 212, 214 and216. As further illustrated, the output optical waveguides of photoniccircuit 200 are coupled to photodetectors 102-108.

In the example of FIG. 2 , couplers 212, 214 and 216 comprisedirectional couplers, where evanescent coupling enables transfer ofoptical power between adjacent waveguides. However, other types ofcouplers may be used such as Y-junctions, X-junctions, opticalcrossovers, counter-direction couplers, etc. In other embodiments,photonic circuit 200 may be implemented with a multi-mode interferometer(MMI). Couplers 212, 214 and 216 may be 3 dB couplers (with a 50%-50%coupling ratio) in some embodiments, though other ratios are alsopossible, such as 51%-49%, 55%-45% or 60%-40%. It should be appreciatedthat, due to fabrication tolerances, the actual coupling ratio maydeviate slightly from the intended coupling ratio.

Signal s₁ may be provided at input optical waveguide 202 and signal s₂may be provided at input optical waveguide 204. Signals s₁ and s₂ may beprovided to the respective input optical waveguides using for exampleoptical fibers. In some embodiments, s₁ represents a reference localoscillator signal, such as the signal generated by a reference laser,and s₂ represents the signal to be detected. As such, the opticalreceiver may be viewed as a homodyne optical receiver. In some suchembodiments, s₁ may be a continuous wave (CW) optical signal while s₂may be modulated. In other embodiments, both signals are modulated orboth signals are CW optical signals, as the application is not limitedto any particular type of signal.

In the example of FIG. 2 , signal s₁ has amplitude A_(LO) and phase ϑ,and signal s₂ has amplitude A_(s) and phase φ. Coupler 212 combinessignals s₁ and s₂ such that signals t and b emerge at respective outputsof coupler 212. In the embodiments in which coupler 212 is a 3 dBcoupler, t and b may be given by the following expression:

$\left( \frac{t}{b} \right) = {\frac{1}{\sqrt{2}}\left( {1i\mspace{11mu} i\; 1} \right)\left( \frac{A_{LO}e^{i\;\vartheta}}{A_{s}e^{i\;\varphi}} \right)}$and the powers T and B (of t and b, respectively) may be given by thefollowing expressions:T=[A _(LO) ² +A _(s) ²+2A _(LO) A _(s) sin(ϑ−φ)]B=[A _(LO) ² +A _(s) ²−2A _(LO) A _(s) sin(ϑ−φ)]

Thus, in the embodiments in which couplers 214 and 216 are 3 dBcouplers, photodetectors 102 and 106 may each receive a power given byT/2 and photodetectors 104 and 108 may each receive a power given byB/2.

Referring back to FIG. 1 , and assuming that the responsivities ofphotodetectors 102-108 are all equal to each other (though not allembodiments are limited in this respect), the currents emerging fromnode 103 and 105, respectively, may be given by the followingexpressions:i _(t) −i _(b)=2A _(LO) A _(s) sin(ϑ−φ)i _(b) −i _(t)=−2A _(Lo) A _(s) sin(ϑ−φ)

DOA 110 is arranged to amplify the differential signal received at the“+” and “−” inputs, and to produce an amplified differential output,represented in FIG. 1 by voltages \T_(out,n) and V_(out,p). In someembodiments, DOA 110, in combination with impedances z, may be viewed asa differential transimpedance amplifier, in that it produces adifferential pair of voltage (V_(out,n), V_(out,p)) based on adifferential pair of current (i_(b)−i_(t), i_(t)−i_(b)). In someembodiments, each of V_(out,n), V_(out,p) may be proportional to thedifference between current i_(t)−i_(b) and current i_(b)−i_(t), thusgiving rise to the following expressions:V _(out,p)=2z(i _(t) −i _(b))V _(out,n)=2Z(i _(b) −i _(t))

This differential pair of voltages may be provided as input to anysuitable electronic circuit, including but not limited to ananalog-to-digital converter (not shown in FIG. 1 ). It should be notedthat optical receiver 100 provides two levels of noise rejection. Thefirst level of noise rejection occurs thanks to the subtraction of thephotocurrents, the second level of noise rejection occurs thanks to thesubtraction taking place in the differential amplification stage. Thisresults in a significant increase in noise rejection.

In the example of FIG. 1 , impedances z are shown as being equal to eachother, however different impedances may be used in other embodiments.These impedances may include passive electric components, such asresistors, capacitors and inductors, and/or active electroniccomponents, such as diode and transistors. The components constitutingthese impedances may be chosen to provide a desired gain and bandwidth,among other possible characteristics.

As discussed above, optical receiver 100 may be integratedmonolithically on a substrate. One such substrate is illustrated in FIG.3A, in accordance with some non-limiting embodiments. In this example,photodetectors 102-108, photonic circuit 200 and DOA 110 aremonolithically integrated as part of substrate 301. In otherembodiments, photodetectors 102-108 and photonic circuit 200 may beintegrated on substrate 301 and DOA 110 may be integrated on a separatesubstrate 302. In the example of FIG. 3B, substrates 301 and 302 areflip-chip bonded to one another. In the example of FIG. 3C, substrates301 and 302 are wire bonded to one another. In yet another example (notillustrated), photodetectors 102-108 and photonic circuit 200 may befabricated on separate substrates.

Some embodiments of the present application are directed to methods forfabricating optical receivers. One such method is depicted in FIG. 4A,in accordance with some non-limiting embodiments. Method 400 begins atact 402, in which a plurality of photodetectors are fabricated on afirst substrate.

Once fabricated, the photodetectors may be connected together, forexample in the arrangement shown in FIG. 1 . In some embodiments, thephotodetectors may be positioned on the first substrate within an areaof 1 mm² or less, 0.1 mm² less or 0.01 mm² or less. At act 404, aphotonic circuit is fabricated on the first substrate. The photoniccircuit may be arranged to provide a pair of optical signals to thephotodetectors, for example in the manner shown in FIG. 2 . At act 406,a differential operational amplifier may be fabricated on a secondsubstrate. An example of a differential operational amplifier is DOA 110of FIG. 1 . At act 408, the first substrate may be bonded to the secondsubstrate, for example via flip-chip bonding (as shown in FIG. 3A), wirebonding (as shown in FIG. 3B), or using any other suitable bondingtechnique. Once the substrates are bonded, the photodetectors of thefirst substrate may be electrically connected to the differentialoperational amplifier of the second substrate, for example in the mannershown in FIG. 1 .

Examples of fabrication processes are depicted schematically at FIGS. 4Bthrough 4G, in accordance with some embodiments. FIG. 4B depicts asubstrate 301 having a lower cladding 412 (e.g., an oxide layer such asa buried oxide layer or other types of dielectric materials) and asemiconductor layer 413 (e.g., a silicon layer or a silicon nitridelayer, or other types of material layers). At FIG. 4C, semiconductorlayer 413 is patterned, for example using a photolithographic exposure,to form regions 414. Regions 414 may be arranged to form opticalwaveguides in some embodiments. In some embodiments, the resultingpattern resembles photonic circuit 200 (FIG. 2 ), where waveguides 202and 204, and couplers 212, 214 and 216 are embedded into one or moreregions 414. At FIG. 4D, photodetectors 102, 104, 106 and 108 (andoptionally, other photodetectors) are formed. In this example, anoptical absorbing material 416 is deposited to be adjacent a region 414.The optical absorbing material 416 may be patterned to form thephotodetectors. The material used for the optical absorbing material maydepend on the wavelength to be detected. For example, germanium may beused for wavelengths in the L-Band, C-Band or O-Band. Silicon may beused for visible wavelengths. Of course, other materials are alsopossible. The optical absorbing material 416 may be positioned to beoptically coupled to regions 414 in any suitable way, including but notlimited to butt coupling, taper coupling and evanescent coupling.

At FIG. 4E, DOA 110 is formed. In some embodiments, DOA 110 includesseveral transistors formed via ion implantation. FIG. 4E depictsimplanted regions 418, which may form part of one or more transistors ofDOA 110. While only one ion implantation is illustrated in FIG. 4E, insome embodiments, formation of DOA 110 may involve more than one ionimplantations. Additionally, DOA 110 may be electrically connected tothe photodetectors, for example via one or more conductive traces formedon substrate 301.

The arrangement of FIG. 4E is such that photonic circuit 200,photodetectors 102-108 and DOA 110 are formed on a common substrate (asshown in FIG. 3A). Arrangements in which DOA 110 is formed on a separatesubstrate (as shown in FIG. 3B or FIG. 3C) are also possible. In onesuch example, DOA 110 is formed on a separate substrate 302, as shown inFIG. 4F, where implanted regions 428 are formed via one or more ionimplantations.

Subsequently, substrate 301 is bonded to substrate 302, andphotodetectors 102-108 are connected to DOA 110. At FIG. 4G, conductivepads 431 are formed and placed in electrical communication with opticalabsorbing material 416, and conductive pads 432 are formed and placed inelectrical communication with implanted regions 428. The conductive padsare electrically connected via wire bonding (as shown in FIG. 4G) or viaflip-chip bonding.

Some embodiments are directed to methods for receiving input opticalsignals. Some such embodiments may involve homodyne detection, thoughthe application is not limited in this respect. Other embodiments mayinvolve heterodyne detection. Yet other embodiments may involve directdetection. In some embodiments, reception of optical signals may involveoptical receiver 100 (FIG. 1 ), though other types of receivers may beused.

An example of a method for receiving an input optical signal is depictedin FIG. 5 , in accordance with some embodiments. Method 500 begins atact 502, in which the input signal is combined with a reference signalto obtain first and second optical signals. The input signal may beencoded with data, for example in the form of amplitude modulation,pulse width modulation, phase or frequency modulation, among other typesof modulation. In some of the embodiments involving homodyne detection,the reference signal may be a signal generated by a local oscillator(e.g., a laser). In other embodiments, the reference signal may also beencoded with data. In some embodiments, the input signal and thereference signal are combined using a photonic circuit 200 (FIG. 2 ),though other types of optical combiners may be used, including but notlimited to MMIs, Y-junctions, X-junctions, optical crossovers, andcounter-direction couplers. In the embodiments in which photonic circuit200 is used, t and b may represent the signals obtained from thecombination of the input signal with the reference signal.

At act 504, the first optical signal is detected with a firstphotodetector and with a second photodetector and the second opticalsignal is detected with a third photodetector and with a fourthphotodetector to produce a pair of differential currents. In someembodiments, act 504 may be performed using optical receiver 100 (FIG. 1). In some such embodiments, the first optical signal is detected withphotodetectors 102 and 106, and the second optical signal is detectedwith photodetectors 104 and 108. The produced pair of differentialcurrents is represented, collectively, by currents i_(b)−i_(t) andi_(t)−i_(b). Being differential, in some embodiments, the currents ofthe pair may have substantially equal amplitudes, but with substantiallyopposite phases (e.g., with a π-phase difference).

At act 506, a differential operational amplifier (e.g., DOA 110 of FIG.1 ) produces a pair of amplified differential voltages using the pair ofdifferential currents produced at act 504. In the embodiments that useDOA 110, the produced pair of differential voltages is represented byvoltages V_(out,n) and V_(out,p). Being differential, in someembodiments, the voltages of the pair may have substantially equalamplitudes, but with substantially opposite phases (e.g., with a π-phasedifference).

Method 500 may have one or more advantages over conventional methods forreceiving optical signals, including for example wider dynamic range,greater signal-to-noise ratio, larger output swing, and increasedsupply-noise immunity.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, and/or methods described herein, if suchfeatures, systems, articles, materials, and/or methods are not mutuallyinconsistent, is included within the scope of the present disclosure.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified.

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value.

What is claimed is:
 1. A method for fabricating an optical receiver, themethod comprising: fabricating first, second, third and fourthphotodetectors on a chip configured to produce first, second, third andfourth photocurrents, respectively, such that an anode of the firstphotodetector is coupled to a cathode of the second photodetector sothat the first photodetector and the second photodetector are configuredto produce a first differential current based on a difference betweenthe first photocurrent and the second photocurrent, and a cathode of thethird photodetector is coupled to an anode of the fourth photodetectorso that the third photodetector and the fourth photodetector areconfigured to produce a second differential current based on adifference between the third photocurrent and the fourth photocurrent;fabricating a differential operational amplifier on the chip havingfirst and second inputs and first and second outputs such that the firstand second photodetectors are coupled to the first input and the thirdand fourth photodetectors are coupled to the second input; andfabricating a photonic circuit on the chip that is configured to providea first optical signal to the first and third photodetectors and asecond optical signal to the second and fourth photodetectors.
 2. Themethod of claim 1, wherein fabricating the first, second, third andfourth photodetectors comprises fabricating the first, second, third andfourth photodetectors such that the anode of the first photodetector andthe cathode of the second photodetector are coupled to the first inputof the differential amplifier.
 3. The method of claim 2, whereinfabricating the first, second, third and fourth photodetectors comprisesfabricating the first, second, third and fourth photodetectors such thatthe cathode of the third photodetector and the anode of the fourthphotodetector are coupled to the second input of the differentialamplifier.
 4. The method of claim 1, wherein fabricating the first,second, third and fourth photodetectors on the chip comprisesfabricating the first, second, third and fourth photodetectors on asilicon-on-insulator substrate or a bulk silicon substrate.
 5. Themethod of claim 1, wherein fabricating the first, second, third andfourth photodetectors comprises fabricating the first, second, third andfourth photodetectors within an area of 0.1 mm² on the chip.
 6. Themethod of claim 1, wherein fabricating the first, second, third andfourth photodetectors comprises fabricating the first, second, third andfourth photodetectors to have equal responsivities.
 7. The method ofclaim 1, further comprising fabricating an analog-to-digital converter(ADC) on the chip such that the ADC is coupled to the first and secondoutputs of the differential amplifier.
 8. The method of claim 1, whereinfabricating the photonic circuit comprises fabricating, on the chip: afirst waveguide coupled to the first photodetector; a second waveguidecoupled to the second photodetector; a third waveguide coupled to thethird photodetector; a fourth waveguide coupled to the fourthphotodetector; a first coupler coupling the first waveguide to the thirdwaveguide; a second coupler coupling the second waveguide to the fourthwaveguide; and a third coupler coupling the first waveguide to thesecond waveguide.
 9. A method for fabricating an optical receiver, themethod comprising: obtaining a first chip comprising: first, second,third and fourth photodetectors configured to produce first, second,third and fourth photocurrents respectively, such that an anode of thefirst photodetector is coupled to a cathode of the second photodetectorso that the first photodetector and the second photodetector areconfigured to produce a first differential current based on a differencebetween the first photocurrent and the second photocurrent, and acathode of the third photodetector is coupled to an anode of the fourthphotodetector so that the third photodetector and the fourthphotodetector are configured to produce a second differential currentbased on a difference between the third photocurrent and the fourthphotocurrent; and a photonic circuit configured to provide a firstoptical signal to the first and third photodetectors and a secondoptical signal to the second and fourth photodetectors; obtaining asecond chip comprising a differential operational amplifier having firstand second inputs and first and second outputs; and bonding the firstchip to the second chip such that the first and second photodetectorsare coupled to the first input and the third and fourth photodetectorsare coupled to the second input.
 10. The method of claim 9, whereinbonding the first chip to the second chip comprises wire bonding thefirst chip to the second chip.
 11. The method of claim 10, whereinbonding the first chip to the second chip comprises flip-chip bondingthe first chip to the second chip.
 12. The method of claim 9, whereinbonding the first chip to the second chip comprises coupling the anodeof the first photodetector and the cathode of the second photodetectorto the first input of the differential amplifier.
 13. The method ofclaim 12, wherein bonding the first chip to the second chip furthercomprises coupling the cathode of the third photodetector and the anodeof the fourth photodetector to the second input of the differentialamplifier.
 14. The method of claim 9, wherein fabricating the first,second, third and fourth photodetectors on the first chip fabricatingthe first, second, third and fourth photodetectors on a siliconphotonics chip.
 15. The method of claim 9, wherein fabricating thefirst, second, third and fourth photodetectors comprises fabricating thefirst, second, third and fourth photodetectors within an area of 0.1 mm²on the chip.
 16. The method of claim 9, wherein fabricating the first,second, third and fourth photodetectors comprises fabricating the first,second, third and fourth photodetectors to have equal responsivities.17. The method of claim 9, wherein the second chip further comprises ananalog-to-digital converter (ADC) coupled to the first and secondoutputs of the differential amplifier.
 18. The method of claim 9,wherein the first chip further comprises: a first waveguide coupled tothe first photodetector; a second waveguide coupled to the secondphotodetector; a third waveguide coupled to the third photodetector; afourth waveguide coupled to the fourth photodetector; a first couplercoupling the first waveguide to the third waveguide; a second couplercoupling the second waveguide to the fourth waveguide; and a thirdcoupler coupling the first waveguide to the second waveguide.